This invention relates to an optical data storage system utilizing a laminate of materials, each exhibiting a different inhomogenous absorption spectrum characteristic, as the storage medium such that each layer of the laminate is responsive to a different wavelength range in a hole burning mode of storage.
U.S. Pat. No. 4,101,976 to Castro et al discloses an optical data storage system that utilizes the frequency or wavelength dimension with a storage material of this type. FIG. 1 shows a system similar to that of Castro et al, wherein the emission wavelength and intensity of a laser 10 are controlled by a scanner 11 in a well known manner. The divergent beam emitted by the laser is collimated by a lens 12 whose parallel beam output is passed through a conventional optical deflector 13 and focused by an objective lens 14 on an addressed memory site or element 16 of a storage medium 15 which exhibits an inhomogenous absorption spectrum as illustrated in FIG. 2A. Storage medium materials of this type are well known in the art, and respond to an incident laser beam of sufficient intensity and within a wavelength range of AB by undergoing an optically induced molecular structural or chemical change by which the addressed site becomes non-absorptive or transparent at the "hole burning" wavelength. Thus, when an addressed memory element 16 of the storage medium 15 is subjected to a focused laser beam of sufficient intensity at the three differently spaced wavelengths as shown by the dotted lines and arrows in FIG. 2A, photoptical holes are burned in the material such that its absorption spectrum exhibits transparency slots or gaps as shown in FIG. 2B. Such gaps or holes correspond to data "1" bits, with data "0" bits being represented by the absence of a hole. Writing is thus implemented by deflecting the focused laser beam to an addressed memory element, scanning its wavelength from A to B, and simultaneously pulsing its intensity at the selected wavelengths to "burn" a 001100100 sequence in the example shown. The storage is essentially permanent, although erasable with some materials, and reading is implemented by similarly scanning the addressed memory element over the wavelength range AB but at a reduced intensity level to prevent any additional hole burning or alteration, which produces a time series output signal from a detector 17 as shown in FIG. 2C.
An obvious limitation with a data storage system of this type is that each memory element can only accommodate a given number of bits as determined by the intrinsic width of the absorption spectrum inhomogeneity and the resolution parameters of the system.